Conventional extrusion of thermoplastic rods such as acrylic rods is normally accomplished by the use of an extruder which typically consists of a screw mechanism rotating within a piston chamber. The action of the screw mechanism mixes and melts the synthetic thermoplastic contained in the piston chamber primarily by the mechanical actions of compression and shearing. The synthetic thermoplastic material, as a result of this mechanical action, becomes plastic and is then able to be squeezed out through a die opening on the outlet end of the extruder which imparts the desired final shape to the rod. Since the extruded synthetic material is above its glass transition temperature (i.e. the temperature at which the synthetic material changes from a solid to a plastic) as it exits the extruder die, it must be cooled immediately as it exits from the extruder die so that the extruded rod retains its extruded shape. Cooling typically is accomplished by blowers, fans and the like which are directed at the die outlet.
Cooling, i.e. by blowing, merely cools the outer periphery of the extruded rod. While this is sufficient so that the rod will retain generally the shape imparted by the extruder die, stresses and voids may be formed internally of the rod as it cools. More particularly, with surface cooling of the rod only the outer periphery initially solidifies. This prevents the rod from contracting across its full diameter as it further cools. Consequently, as the internal plastic molecules take up less volume after they have cooled and shrunk, the remaining empty space in the interior of the rod is filled by stretching of the remaining molecules. The net effect of this non-uniform cooling is that the outer wall of the rod sets first and becomes fixed (because it has solidified) while the interior plastic material continues to contract as it cools and solidifies. This uneven cooling can result in the formation of voids, bubbles and interior molecular stresses in the interior of the extruded product. This phenomena is particularly acute in the case of rods formed of acrylic due to inherent good heat insulating characteristics of acrylic molecules. A major problem which results from the presence of interior bubbles or voids and molecular stresses is that a fracture or chip in the rod can occur when the rod is drilled, machined or otherwise manipulated.
The contraction problem becomes more significant as the diameter and wall thickness of the rod increases since the amount of contraction of the interior plastic material is directly related to the diameter of the rod. For example, in the case of rods formed of acrylic materials having diameters of approximately 13/4", the amount of diametric stretching within the interior of the rod becomes so great that the interior molecules typically can no longer withstand the stress. The molecules thus may tear apart from one another during contraction and form bubbles or voids within the rod. As a result, there heretofore has been a practical limitation upon the maximum diameter that extruded acrylic rods can be acceptably produced at normal production rates.